I've seen the National Guard use them as tactical radios, in the airport post 9/11, this was still 2001 and 2002 though, so the bubble pack phenomenon hadn't fully taken off yet. I yanked one of my FRSs out of my carry on and listened in.
It would not surprise me to hear municipalities using these improperly on the relatively quiet GMRS frequencies (unless you know a repeater most people I've heard when scanning are on FRS Ch1, out of lazyness) esp as the cheap quick fix to not having a tactical channel available for events like fairs and such.
That said you generally *won't* find that to be the case cause it undermines the whole agency when it comes to public opinion and law enforcement.
My vote would be some kids playing around, but you said they sounded professional, so maybe the rebroadcast theory has some merit, or someone didn't know what they were doing when the programmed the new UHF business radios, many of which carry GMRS frequencies if you program them in there. Again this is something I'd expect from a small department, with no budget for any sort of staff educated about radios, which is very uncommon.
Obviously this is all speculation on what you heard, I'd listen up and see if you can find them again, then see if you can find them on a different frequency with the scanner, and try and figure out what just agency it is.